Typically, only a limited amount of power is available from the power supply of a hearing aid. For example, a conventional button cell Zinc-air battery typically supplies power to a hearing aid circuit due to its high energy density and low cost.
In the design of a hearing aid, the size and the power consumption are important considerations. The battery is a large component of the hearing aid, and to ensure compact and inconspicuous hearing aids, small sized batteries, such as the “312” and “13” types are used. However, small batteries have a relatively large internal resistance. For example, a “312” battery typically has an internal resistance of 5Ω-10Ω compared to typical internal resistance values of 0.1Ω-0.5Ω of an AA type battery. The large internal resistance causes the supply voltage to drop significantly as a function of increased output current. Voltage drops may result in reduced sound quality and/or interrupted operation of parts of the hearing aid.
A radio chip for wireless communication draws significant amounts of current during on-going transmission and reception. A conventional Zinc-air battery is only capable of supplying the required current for wireless transmission and reception for a limited time period, typically 0.5-5 milliseconds (ms). If the battery continues to supply the required amount of current for longer time periods, the supply voltage will decrease, and below a certain threshold, the hearing aid circuit, in particular digital parts of the hearing aid circuit, will not operate properly.
Further, Zinc-air batteries require time to recover after having supplied peak currents, even for limited time periods. Typically, the radio chip duty cycle, i.e. the percentage of radio turn-on time with respect to the sum of the radio turn-on and radio turn-off time, must be kept below 15%-20%.